1. Field of the Invention
The invention relates to flat glass sheets with differentiated entiated surface hardening or tempering which can be used as automobile windshields.
2. Description of the Prior Art
The desire to preserve adequate visibility for drivers if their windshield should break has resulted in the provision of a central zone of relatively small dimensions which is not hardened or only slightly hardened in an otherwise hardened windshield which fragments into small pieces when broken. This central zone does not fragment and allows visibility in spite of the breaking of the entire windshield section which surrounds this central zone.
In order to obtain such a result, the central zone and a peripheral zone are subjected to different thermal conditions. In order to avoid fragility of a glass sheet formed in this manner, due to different thermal treatments, French Pat. No. 1,038,439 proposes differentiating the heating of the two zones before a uniform hardening, whereby the central zone is less heated during hardening than the peripheral zone, on account of screens introduced into the furnace in such a manner that the glass of this central zone is at a temperature which is lower or at the most equal to that of the surrounding zone. As a variant, the entire glass sheet is heated in a uniform manner but the cooling of different portions is differentiated by accelerating the cooling of the central zone in such a manner that it solidifies before the surrounding peripheral zone. The cooling of this central zone is nevertheless slow enough that it hardens only a little or not at all.
However, such glass sheets with a slightly or non-hardened zone do not meet European Government regulations concerning flat automobile windshields, which require that the entire windshield be hardened, even if the hardening is different at the center and at the periphery.
Such regulations state that after breaking, a visibility of at least 15% of full visibility remains in a central zone of at least 20 cm.times.50 cm, the center of which is approximately in front of the driver; that is, that the total of the surfaces of all the fragments of at least 2 cm.sup.2 must be at least 15%, and that no fragment may have a surface area greater than 16 cm.sup.2. They also require that the fragmentation in the peripheral zone be fine, that is, with 40 to 350 fragments in a square 5 cm.times.5 cm and that there are no needles greater than 7.5 cm in length in this same peripheral zone.
Other patents teach performing hardening processes which cover the entire glass sheet but are differentiated according to the zones.
Thus, French Pat. No. 1,150,913 proposes a device for differentiated hardening comprising a blast box with distinct chambers opposite the various zones of the glass sheet to be hardened. These chambers communicate with the box via regulatable air entry orifices in order to adapt the amount of air arriving at the blast nozzles of the different chambers to the desired hardening effect.
French Pat. No. 1,259,550 analogously proposes cooling the hot glass sheets with jets of air, the blast pressure of which is less in the central part than at the periphery of the sheet.
French Pat. No. 1,339,535 describes a slightly different type of windshield having, like the preceeding ones, a peripheral zone of fine fragmentation and a central zone of coarse fragmentation, but in which the central zone is divided vertically by fine bands of finer fragmentation. The cooling of the glass sheet in such vertical bands is performed by blowing a gas on its two faces, and an oscillation in a vertical plane can be given to either the nozzles or to the glass sheet.
The applicant has determined that such methods are not entirely satisfactory, especially in the case of flat glass sheets, which acquire an undesired curvature and in which the differences in hardening between the center and the periphery induce alterations of the sought-after fragmentation pattern, which risks making these glass sheets unfit for automobile windshields.